A Roberval load cell is used for, for example, a balance for commerce and industry, and includes a flexure element having four thin parts in all which are composed of a top pair and a bottom pair each having two thin parts. The base part of the flexure element is fixed in a cantilever manner, and the top part is applied with a load. Two thin parts out of the four parts act as tension sides and another two parts act as compression sides. Strain gauges are bonded to each of the thin parts of the tension sides and of the compression sides. The four strain gages are connected with one another to constitute the Wheatstone bridge.
In the load cell having such configuration, a stopper for preventing the overload is mounted for preventing the excessive deformation of the thin parts during the application of the overload to the load cell (flexure element).
Ordinarily, the stopper is mounted in the vicinity of the load cell separated from the load cell itself, and, for example, a screw-type separate stopper is mounted between a support frame by which the load cell is vertically fixed and a bottom plate of a housing in a commercial desktop balance (for example, refer to Patent Publication 1).
This separate stopper is required to be mounted depending on a kind of a load. For example, one or two stoppers are mounted on the front end of the load cell in case of a vertical lower load (that is, a load when a subject to be measured is dropped on or near the center of a weighing dish). Four stoppers are mounted on every four corners of the load cell in case of a torsional and vertical lower load (a load when a subject to be measured is dropped on or near the four corners of a weighing dish). The former stopper is referred to as a center stopper while the latter is referred to as a four-corner stopper.
Further, a stopper for an upper load is required in addition to the lower load, and the stopper for a vertical upper load and a torsional and vertical upper load is required. The vertical upper load herein is an overload which, for example, when a vertical overload is applied (or a subject to be weighed is dropped on a balance) and the stopper receives a downward load, is generated by bouncing the load cell in an upward direction by means of a shocking reactive force of a load and reversely transmitted in an upward direction. In another case, the vertical upper load is generated when a weighing apparatus is conveyed by grasping a weighing dish.
On the other hand, the torsional and vertical upper load is an overload which, when a subject to be weighed is dropped on the vicinity of one of the four corners of the weighing dish so that the torsional load is applied on the load cell followed by the generation of a downward load on the side the subject is dropped, is transmitted by twisting the other side of the dish reversely in an upward direction. Also in case of these upper loads, stoppers are required. In case of separate stoppers, the stopper is required for each type of the stoppers so that a problem arises that the number of the stoppers increases.
The separate stopper includes problems of lower development efficiencies, lower assembling efficiencies and lower processing efficiencies of the load cells. Specifically, the separate stopper is arranged keeping a specified clearance with respect to the flexure element of the load cell. Since this clearance is determined by joining a warp of the load cell during the loading and a warp of a supporting member of the load cell such as a supporting frame and a housing bottom plate, this clearance can be determined only after the shape of the balance is determined and an experiment is conducted. Accordingly, there arises a defect that a long period of development is required and a development efficiency is low.
Additional time is required for the assembling of the load cell because the assembling is conducted while the clearance is determined by using a clearance gauge. The separate stopper includes a defect of not completely preventing of the overload delivered to the load cell because of the deformation of the supporting member for the load cell during the overload.
A stopper-integrated load cell having a built-in stopper is proposed for overcoming these defects.
For example, in Patent Publication 2, horizontally coaxial circular apertures are formed in a fixed portion and a movable portion of a flexure element, and a circular cylindrical stopper fitted and fixed to the circular aperture of the fixed portion is inserted into and disposed in the circular aperture of the movable portion. In this load cell, the deformation of the flexure element due to the overload can be prevented by the contact of the inner circumferential surface of the circular aperture of the movable portion with the outer circumferential surface of the stopper when the overload is applied from side to side and up and down. The simple structure including only the circular cylindrical stopper enables the simple processing and assembling of the load cell.
FIGS. 6 and 7 of Patent Publication 3 depict a stopper horizontally extending and having a square section and a load cell having a penetration aperture. The load cell of Patent Publication 3 also enables the simple processing and assembling of the load cell.